The field of the invention relates generally to accelerometry. More specifically, the field of the invention relates to a microelectromechanical accelerometer. In particular, the field of the invention relates to a microelectromechanical accelerometer fabricated from single crystal silicon, with improved performance qualities, for use in automotive and related applications, and a manufacturing method for making such accelerometers at low cost.
There is a need for automotive safety systems to collect more information about vehicle dynamics and external forces acting on the vehicle in order to make intelligent decisions as to what actions, if any, need to be taken to maintain safe vehicle operation. Collecting such information is the role of sensors, such as accelerometers, force sensors, pressure sensors, and the like. With presently available sensor technologies, only a limited number of sensors can be utilized in a vehicle before their cost becomes prohibitively high. What is required is a new, high-performance, low-cost technology for automotive sensors. Silicon-based devices and microelectronics-style manufacturing techniques are anticipated to be required to meet the price-performance objectives of automotive sensors in the future. See G. A. MacDonald, "A Review of Low Cost Accelerometers for Vehicle Dynamics," Sensors and Actuators A21-A23 (1990), pp. 303-307; and Robert E. Sulouff, Jr., "Silicon Sensors for Automotive Applications," Proc. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers '91), San Francisco, Calif., Jun. 24-28, 1991, pp. 170-176.
There are numerous applications for accelerometers in automobiles, including airbag deployment (front, rear, and side impact), anti-lock brake systems, roll detection, angular rate accelerometers, electronically controlled suspension systems, steering systems, and collision avoidance systems, to name a few. Each application requires accelerometers which operate in different ranges of acceleration (from as little as 10.sup.-6 g to as much as 500 g) and bandwidth, yet all with stringent requirements on reliability, operating environment, self testability, and cost. See G. A. MacDonald, "A Review of Low Cost Accelerometers for Vehicle Dynamics," Sensors and Actuators A21-A23 (1990), pp. 303-307; and Robert E. Sulouff, Jr., "Silicon Sensors for Automotive Applications," Proc. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers '91), San Francisco, Calif., Jun. 24-28, 1991, pp. 170-176.
Introduction of new technology to automotive applications is primarily driven by price-performance considerations. Although more intelligent safety systems are desired, the cost of those systems must continuously drop while their performance improves. If improving the system's performance requires more sensors, the price of individual sensors and their associated electronics must be correspondingly lower. Assuming a ten to twenty times increase in the number of sensors (not unreasonable considering a fully active suspension is predicted to require ten accelerometers alone) and a ten times reduction in the cost of the overall safety system, then the sensors themselves must be produced for less than one-hundredth (1/100) their current price. As a concrete example, high performance piezoelectric quartz accelerometers which could be utilized in these automotive applications currently retail for $300. The automobile industry predicts that such sensors will not be incorporated into production vehicles until a technology can be found which can supply the desired sensor for $2 to $3 per unit.
An excellent example of the commercial reality discussed above can be found in automotive airbag systems. Over the past five years a concerted effort in the industry has been made to develop a new airbag deployment subsystem costing less than one-tenth that of the current technology. Substantial investment in time and research funds have been made and functioning devices have been delivered to potential customers. However, these devices cannot meet the cost targets set out for them by the automotive industry. Hence, despite much promise, there is at present virtually no use of these new accelerometer devices in production airbag systems. The following discussion of accelerometers, and particularly micromachined accelerometers, helps to explain why these accelerometers have high cost and have not achieved widespread use in automotive applications.
An accelerometer in general is a device which senses an externally-induced acceleration. There are three major components to an accelerometer, as shown in FIG. 1. Typically, a sense element is a mass of some sort which moves in response to an applied acceleration vector. This is referred to as a mass, proof mass or seismic mass. The proof mass is held in its resting position by a spring. Some form of displacement transducer is used to measure the amount of motion the proof mass makes in response to an applied acceleration. This is then converted into an electrical output signal and may include signal conditioning electronics to provide a strengthened signal for accurate measurement of the displacement. The output signal from signal conditioning electronics then may be used by additional electronic control circuitry to determine how to respond to the detected acceleration. For example, a charge may be activated for deploying an airbag in response to a sensed acceleration vector above a pre-determined threshold. See Ernest O. Doebelin, Measurement Systems: Application and Design, (McGraw-Hill, New York, 1990), Chapter 4.6, incorporated herein by reference.
Present generation production airbag deployment sensors utilize physically "large" mechanical devices, such as a metallic ball held between the poles of a permanent magnet, as the accelerometer to detect impact (deceleration) of sufficient magnitude to signal deployment of the airbag, typically an impact in excess of 50 g (490 m/s.sup.2). See, for example, U.S. Pat. No. 5,098,122. This type of conventional accelerometer has severe disadvantages in terms of cost, reliability, sensitivity, and self-testing ability. Thus, there is a compelling need for an alternative accelerometer technology for an airbag deployment system which provides low cost, reliable and ultra sensitive operation along with self testing capability.
Moreover, there are numerous other applications for accelerometers in automobiles such as active suspension, anti-lock braking, and active steering, and the necessary broad range of operating characteristics for active steering which cannot be met by current "large" mechanical accelerometer technology. Solid-state accelerometers based on the piezoelectric effect in many cases have been implemented in an attempt to meet the performance requirements of these additional applications. However, such conventional piezoelectric accelerometers, are too expensive and/or physically too large to be practical for implementation in automobiles. See, for example, U.S. Pat. No. 4,945,765, which notes that these large accelerometers can be several cubic inches in size and weigh a pound.
The emerging technology of micromechanical systems (MEMS) has created an entirely new approach to accelerometers (see, for example, Janusz Bryzek, Kurt Petersen, and Wendell McCulley, "Micromachines on the March," IEEE Spectrum, May 1994, pp. 20-31, and Lee O'Connor, "MEMS: Micromechanical Systems," Mechanical Engineering, Feb. 1992, pp.40-47). Numerous patents have been issued for a variety of micromechanical accelerometers over the past fifteen years (for example, U.S. Pat. Nos. 4,483,194; 4,553,436; 4,736,629; 4,945,765; 5,126,812; 5,249,465; and 5,345,824). The earliest of these patents, as well as research papers from the late '70's made reference to the potential application of micromechanical accelerometers in automotive applications based on the potential of MEMS to meet both the cost and performance requirements described above. Yet as far as is known at this time, few MEMS accelerometers are used in automotive applications.
MEMS utilizes microelectronic processing techniques to reduce mechanical components to the scale of microelectronics. In some cases, it is even possible, although quite difficult, to place both the mechanical components and electronics onto a common silicon chip. MEMS offers the opportunity for integrating mechanical sensor elements and their associated signal processing electronics onto a single chip in a common manufacturing process, if a viable process for this integration can be found. This integrated approach is in stark contrast to existing technology in which separate manufacturing processes and facilities must be used to fabricate the mechanical components and the electronic components. Those individual components then must be assembled together in the final package. This results in manufacturing complexity and greatly increases the cost of the final product. Consequently, MEMS offers the potential for substantial reductions in size and weight, and tremendous improvements in cost, performance, and reliability when compared to existing technology.
Two principal fabrication technologies are used to create MEMS devices: bulk and surface micromachining. See, for example, U.S. Pat. Nos. 4,736,629 and 5,345,824; Theresa A. Core, W. K. Tsang, and Steven J. Sherman, "Fabrication Technology for an Integrated Surface-Micromachined Sensor," Solid State Technology, October 1993, pp. 39-47; and Wolfgang Kuehnel and Steven Sherman, "A surface micromachined silicon accelerometer with on-chip detection circuitry," Sensors and Actuators A 45 (1994), pp. 7-16; Frank Goodenough, "Airbags Boom When IC Accelerometer Sees 50 g," Electronic Design, Aug. 8, 1991, pp. 45-56; Lynn Michelle Roylance and James B. Angell, "A Batch-Fabricated Silicon Accelerometer," IEEE Trans. Electron Dev. ED-26 (1979) pp. 1911-1917; Lj. Ristic, D. Koury, E. Joseph, F. Shermansky, and M. Kniffin, "A Two-Chip Accelerometer System for Automotive Applications," Proc. MicroSystem Technologies '94, Berlin, Oct. 19-21, 1994, pp. 77-84; and U.S. Pat. No. 5,249,465.
Bulk Micromachining
Conventional MEMS technology which relies upon a bulk micromachining process to produce structures in silicon has disadvantages for accelerometer applications.
Bulk micromachining utilizes certain chemical etchants, most notably aqueous potassium hydroxide (KOH and aqueous ethylenediamine pyrocatechol (EDP), which preferentially etch along certain crystal planes in silicon. These chemicals can be used to sculpt out certain geometric structures in silicon.
Since bulk micromachining uses wet chemicals to etch preferentially along certain crystallographic planes in silicon, this has the disadvantage of limiting the shapes of the structures to ones that correspond to those atomic planes, typically pyramidal structures.
The depth of the etch is determined by the chemical composition of the etching solution, the localized conditions under which etching is performed (e.g. temperature, concentration, extent of turbulence or convection, etc., at the particular location and crystal plane being etched), and the time allowed for etching to take place. Mask materials, such as silicon dioxide or silicon nitride, are used to protect areas of the silicon which are not to be etched, although these areas may be undercut by the etchant under certain conditions.
Bulk micromachining is very limited in terms of the resolution, accuracy, and repeatability with which it can define structures and the geometries of those structures. Etch rates vary with the type of crystal plane being etched, and exact and precise control of reaction conditions and times is essential to obtain a micromachined device of the geometry desired. The process is, furthermore, extremely difficult to integrate with conventional microelectronic device fabrication techniques, thereby essentially precluding the integration of the mechanical components and electronic devices on a single chip.
Most often bulk micromachining techniques require bonding a second silicon wafer or other substrate material to the original wafer containing the etched structures. FIG. 2 illustrates one such bulk micromachined accelerometer. The sense element is formed by chemically etching a first silicon wafer from the backside to remove material, leaving a trapezoidal block of silicon, the mass, suspended by thin silicon membranes on either side of it. The depth of etch in this example is controlled by first implanting the front side of the silicon wafer with an etch-stop layer, typically boron, at the desired depth. When the chemical etchant encounters this layer, its etch rate drops to almost zero. The silicon membranes are thin enough that they allow the mass to move in the vertical direction (out of the plane of the wafer) under applied acceleration. The membranes serve the function of a spring.
Referring to FIG. 2, a second silicon wafer, etched in a similar manner, is then bonded to the first to form the cavity in which the mass resides. This second wafer also provides overrange stops (not shown) to prevent the mass from moving too far and damaging itself. A third wafer (not shown) similar to the bottom one can be bonded on top to provide overrange protection in the other direction, as well as a self-test ability. See Henry V. Allen, Stephen C. Terry, and Diederik W. De Bruin, "Accelerometer Systems with Built-in Testing," Sensors and Actuators A21-A23 (1990), pp. 381-386, and Lynn Michelle Roylance and James B. Angell, "A Batch-Fabricated Silicon Accelerometer," IEEE Trans. Electron Dev. ED-26 (1979), pp. 1911-1917.
The conventional bulk micromachined accelerometer as shown in FIG. 2, is difficult to manufacture. Three different wafers must be etched and processed, then all are bonded together. Bonding the three wafers is a difficult task and only can be performed at a high temperature. Such a high temperature process precludes the inclusion of any electronics on any of the wafers involved. Thus, a second chip must be used to carry the electronics. The number of process steps, wafers, and calibration of the piezoresistors (trimming) results in a very expensive and relatively complex device.
Control over the wet chemical etch is difficult in a bulk micromachining manufacturing process in terms of run-to-run repeatability and uniformity across a wafer. Ion implanted etch stops alleviate some of the uniformity issues; however, they introduce high temperature annealing stresses and have a very limited range of depths to which the implant can reach. Since the etching process is incapable of creating a released structure, such as a cantilevered beam over a surface, a second wafer or other material such as glass or metal must be bonded to the etched wafer to create a released structure.
Thus, bulk micromachining precludes the formation of a highly desirable feature such as a released structure, for example, a released cantilevered beam over a surface. Such released structures are necessary to form electrical contacts beneath certain micromechanical structures for capacitive sensing, actuation, and the like.
Therefore, it would be highly desirable to be able to fabricate a released structure such as a cantilevered beam over a surface for accelerometer applications. Such an accelerometer would be extremely compact and sensitive to an applied acceleration. In addition, the cantilevered beam would provide an inherent spring for returning the beam to a stable sensing position.
Bulk micromachining also requires wafer bonding which limits the ability to integrate such an accelerometer with signal conditioning circuitry or other electronics which are required for production of an accurate output signal. Wafer bonding is a high temperature process which precludes having pre-existing electronic devices on any wafer involved. It further creates stresses in the structures which can lead to other problems or device failure. It is, furthermore, an additional process step which adds to the cost of the device.
Surface Micromachining
To overcome some of the problems associated with bulk micromachining, various researchers developed a process known as surface micromachining. In surface micromachining, a sacrificial layer of silicon dioxide ("oxide"), or other appropriate material, is first deposited on the surface of the silicon wafer. A second layer consisting of polycrystalline silicon ("poly") is then deposited on top of the oxide. The poly is patterned into desired mechanical structures using conventional semiconductor processing techniques. Finally, the oxide is etched out from under the poly, leaving the mechanical structures free to move. This process solves the resolution and structure geometry limitations of bulk micromachining.
FIG. 3 illustrates a conventional accelerometer formed with surface micromachining. Such a surface micromachined accelerometer is exemplified by U.S. Pat. No. 5,345,824. The sense element (mass), which consists of an array of fingers extending from a backbone, is formed from the deposited polysilicon layer and is suspended from tethers at either side. The ends of the tethers are attached to an underlying silicon substrate (not shown for clarity), as are the fixed electrodes. The tethers provide the spring function shown for the generic accelerometer of FIG. 1.
Surface micromachining, however, has a number of limitations of its own in terms of practical applications. The height of the mechanical devices formed by surface micromachining is determined by the thickness of the poly layer in which devices are formed. That thickness is in turn limited by stresses which are inherently developed in the poly film during the deposition process. The practical limitation on such film thickness before catastrophic failure is between two and four micrometers (2-4 .mu.m). Even when the poly layer is held to less than these limits, stresses can result in films which are not planar relative to the substrate wafer. Also, such films may be characterized by a planarity which varies from one deposition run to the next, resulting in non-reproducible results in manufacturing. As will be described in detail below, the small height of the mechanical structures (&lt;4 .mu.m) formed in surface micromachining severely limits the performance of MEMS devices fabricated in this manner. These limitations have particular negative impact on the ability of this technology to meet the requirements of automotive accelerometers.
Another major limitation to surface micromachining relates to manufacturability. The release of the mechanical structures must be obtained by removing the sacrificial layer out from under the poly. That release step is most often accomplished with a wet chemical etchant which will selectively remove oxide, notably hydrofluoric acid (HF), followed by a rinse to remove any acid residue. During this procedure the surface tension (capillary) forces present are often of sufficient magnitude to cause the thin poly structures to be pulled down into contact with the wafer substrate and to stick in that position, thereby rendering the device unusable. This problem is sufficiently widespread that it has been given the name "stiction" and is the subject of several research papers. Failure by stiction reduces the manufacturing yield of surface micromachined MEMS devices, thereby increasing their unit costs.
U.S. Pat. No. 5,314,572 discusses at length the problem of stiction. The '572 patent presents a complex two stage process to attempt to avoid stiction. The '572 patent points out that one needs to use a plasma etch to do the release step. However, since the '572 patent teaches the use of polysilicon as the mechanical material, plasma etching cannot be used to remove a sacrificial oxide layer as the oxide etch is not selective enough over polysilicon and such a process would consume too much polysilicon. Thus, an objective of the '572 patent is to end up with a photoresist "polymer" sacrificial layer which can be plasma etched with sufficient selectivity over polysilicon. However, one cannot start with a polymer sacrificial layer since the deposition temperature (600.degree. C.) of polysilicon is far higher than a polymer can withstand. Thus, a complex manufacturing process is required: making a first sacrificial layer of oxide, then depositing polysilicon, etching away the oxide chemically, returning and filling holes with photoresist, finishing the micromachined structure, and lastly using a plasma etch to remove photoresist supports holding up the polysilicon structures to be released. Such a manufacturing process is complex, suffers from problems of reproducibility and increased cost. In addition, it is not clear that such a process overcomes entirely the problem of stiction.
Integration With Displacement Transducer Circuitry
A further problem of the surface micromachining process is its limited ability to be integrated with conventional microelectronics device processing techniques. The wet chemical etch associated with the release step and the moderately high temperature (approximately 600.degree. C.) of polysilicon deposition are incompatible with aspects of microelectronic device fabrication (e.g. implanted boron migrates readily at 600.degree. C., so implantation and other electronics fabrication cannot occur until after polysilicon is deposited). This problem results in a very complex process if both mechanical and electronic devices are to be fabricated on a single chip. Such complexity results in low yields in manufacturing and thus high unit costs.
An approach to avoid integration problems is to use two chips, one for the surface micromachined sense element and transducer, and one for the signal conditioning electronics. See L. Ristic, D. Koury, E. Joseph, F. Shermansky, and M. Kniffin, "A Two-Chip Accelerometer System for Automotive Applications," Proc. MicroSystem Technologies '94, Berlin, Oct. 19-21, 1994). Although the two chip approach avoids the problems of integrating surface micromachining with conventional microelectronic device processing, it results in a larger, more complex, and more costly accelerometer.
In solid-state and micromechanical accelerometers, two displacement transducers are commonly used, piezoresistive and capacitive. The transducers provide a means for converting the movement of the proof mass or inertial mass into an output signal representative of the sensed acceleration. In piezoresistive transduction, motion of the mass is transduced by the change in resistance of a piezoresistive material as it is deformed (expanded or contracted). Such a device can be readily created by ion implantation of an appropriate dopant into thin sections of silicon attached to the mass (see FIG. 1 for example). The major limitation to piezoresistive schemes is that the piezoresistor is strongly influenced by temperature. Hence, costly compensation electronics are required to mitigate the effects of changes in temperature on the device's performance. Piezoresistors are also sensitive to stress which may be introduced during processing or mounting of the device in its package or on the vehicle.
To alleviate the problems associated with piezoresistive transducers, most accelerometer devices today utilize a capacitive transducer. In this approach, motion is transduced by having that motion alter the capacitance of a structure. Changes in capacitance are readily measured electronically and are relatively unaffected by changes in temperature. This technique is illustrated in FIG. 3 for the surface micromachined accelerometer. Motion of the suspended polysilicon mass changes the spacing between the fixed and suspended electrodes, thereby enabling a measurement of the change in capacitance as a function of the mass' motion.
Many bulk micromachined accelerometers use a second wafer bonded to the first to obtain an electrode beneath the bulk micromachined movable structure. See U.S. Pat. No. 4,483,194 and Kurt E. Petersen, Anne Shartel, and Norman F. Raley, "Micromechanical Accelerometer Integrated with MOS Detection Circuitry," IEEE Trans. Electron Dev. ED-29 (1992), pp. 23-27. A serious drawback to the design presented in '194 as compared with that shown in FIG. 3 is the highly nonlinear variation in capacitance with motion which results from '194's cantilevered beam, or "flap." This design is further limited in terms of manufacturing as it requires bonding of two wafers, and the device responds to acceleration perpendicular to the plane of the wafer. This makes mounting the device in the vehicle more difficult. The preferred method is to sense acceleration in the plane of the wafer.
Another example of conventional capacitive transduction with bulk micromachining is provided in U.S. Pat. No. 4,736,629. In this instance, a silicon wafer is processed using conventional microelectronics device fabrication techniques to form the fixed electrodes and the signal conditioning electronics. The wafer is then covered with a passivation layer (e.g. glass) which is patterned with lithography and chemical etching. Metal is deposited on the passivation layer, portions of which are then etched out from under the metal, leaving a suspended metal structure which forms the mass and the movable portion of the capacitor. The resulting device consists of a metal plate having two movable electrodes of two separate capacitors supported by a metal pedestal above the surface of an insulated silicon substrate that also has two fixed electrodes which, in conjunction with the electrodes on the metal plate, form the two capacitors used in the disclosed accelerometer. The capacitance of one capacitor is compared to the capacitance of the other capacitor, and the difference between these capacitances is used to determine the acceleration.
The accelerometer of the '629 patent has a single moving element, the metal plate. This has the disadvantages of very small capacitance and a correspondingly small output signal. Parts formed from metal have inferior fatigue properties when contrasted with silicon. Furthermore, the change in capacitance with motion is highly nonlinear (see also the above paragraph on nonlinear capacitance for a suspended flap). The device has a very small output signal and requires a second chip of electronics to process the output signal into a signal that can be transmitted to and used by other electronic components, such as the electronics that trigger deployment of an airbag. This device also measures acceleration perpendicular to the plane of the wafer and therefore suffers from the problems associated with this method, as discussed previously.
Problems of Conventional Displacement Transducers
A further disadvantage of conventional accelerometers is the difficulty in integrating the transducer and associated signal conditioning circuitry on a single chip.
U.S. Pat. No. 5,345,824 discloses an acceleration sensor and signal conditioning circuitry that is said to be formed on the same chip on which the sensor is placed. U.S. Pat. No. 5,314,572 discloses the method of making this accelerometer. The accelerometer comprises polycrystalline silicon ("poly" or "polysilicon") and fixed and movable beams forming two capacitors that are suspended above the surface of a silicon substrate by polysilicon posts. Two capacitors are formed above the surface, and the difference in capacitance between the two capacitors is used to determine the acceleration. The process of making the accelerometer begins by depositing a sacrificial oxide layer on a silicon wafer and etching holes in the oxide layer where the polysilicon support posts are to be. Polysilicon is then deposited onto the oxide layer requiring a temperature of approximately 600.degree. C.
This has the problem of precluding fabrication of many electronics components on the wafer (at least to this point in the process). The polysilicon is then etched, and oxide is subsequently etched using a liquid isotropic etch designed to undercut the poly layer. A photoresist is deposited and etched to provide short photoresist posts that support the suspended capacitive fingers and prevent "stiction" (i.e. bending of the suspended fingers and adhesion to the substrate surface) in subsequent wet etch, rinse, and drying steps that remove the sacrificial oxide layer. Any remaining photoresist is removed via, e.g., oxygen-plasma stripping. Presumably, once the sensor has been fabricated, associated electronics can then be etched into the surface of the wafer and connected to the sensor. This process suffers from problems of manufacturing complexity, associated high cost and stiction, or bonding of moving components as previously described.
U.S. Pat. No. 5,345,824 also illustrates other problems associated with a conventional surface micromachining process. The thickness of the polysilicon which can be deposited is limited to approximately 3-5 microns. Thus, the aspect ratio of structures which can be formed using surface micromachining is very limited. Aspect ratio (or beam height) plays a critical role in providing sufficient capacitance to detect minute deflections of the sensing element or proof mass induced by varying or low levels of acceleration. For example, the '824 patent states that the device capacitance available is very small, with total device capacitance being approximately 0.1 pF [col. 5, line 1-2, and 6]. An inventor of the '824 patent shows that one-half of the transducer signal is lost because the lowest input capacitance signal conditioning electronics available has approximately 0.1 pF of stray capacitance. That is, on the same order as the transducer itself. This results in a 6 dB or one-half reduction in signal strength. Such a device lacks essential sensitivity to minute deflections induced by varying acceleration levels. Accordingly, a device as taught by the '824 patent would be unsuited for applications requiring extreme sensitivity to low level accelerations at low frequencies.
For example, in electronic brake distribution (EBD) systems, relatively low-level, wheelslip signals available during braking must be sensed in order to modify brake pressures for optimized distribution. A conventional accelerometer would be too insensitive for applications such as active steering, wherein the output signals of an extremely accurate accelerometer would be needed to alter vehicle handling dynamics such as by applying signals for modifying brake torque to actively achieve a desired objective such as skid avoidance.
Conventional accelerometers lack the sensitivity to detect minute changes in acceleration or to distinguish accurately dynamically changing acceleration vectors such as tilt, inertia, shock or vibration. In an attempt to overcome problems of insensitivity, U.S. Pat. No. 4,711,128 discloses a multi-finger capacitive accelerometer that appears to have been fabricated from a thin slice of quartz. The patent is silent on how the accelerometer was fabricated, as is U.S. Pat. No. 4,663,972 to which '128 refers, other than to say that the accelerometer was "micromachined". Semiconductor devices are not fabricated from quartz, an oxide of silicon, so it would not be possible to produce a wafer having the accelerometer and electronics on a single substrate. An accelerometer of '128 would be expected to be extremely delicate and therefore have very low yields when manufactured or attached to a silicon substrate containing the electronics to process the signal from the '128 accelerometer.
U.S. Pat. No. 4,483,194 discloses an accelerometer that requires two layers that are glued to each other, each of which carries an electrode that together form a capacitor. The accelerometer comprises a first layer of a silicon substrate from which the first electrode is fabricated via, e.g., reactive ion etching, and a second layer such as glass that carries the second electrode. This multi-layer device measures the change in capacitance caused by the capacitor plate on the flap or vane on the substrate flexing about resilient attachment means such as pivots. The multi-layer accelerometer is expected to have disadvantageously very small changes in capacitance, and it also measures accelerations normal to the plane of the substrate or otherwise out of the plane of the substrate.
U.S. Pat. No. 5,249,465 discloses an accelerometer having a polysilicon seismic mass supported above a silicon wafer by polysilicon springs and pedestal. Fixed polysilicon electrodes are anchored to the substrate and are suspended above the seismic mass by polysilicon supports. A pair of capacitors are formed, comprising (1) a fixed electrode on the polysilicon suspended above the seismic mass and a movable electrode on the upper face of the polysilicon seismic mass; and (2) a fixed electrode on the silicon wafer and a movable electrode on the lower face of the polysilicon seismic mass. Pairs of electrodes are distributed about the seismic mass and are used in conjunction with springs to maintain the seismic mass in a neutral position. This accelerometer requires deposition of polysilicon at temperatures that are incompatible with electronic components that might have been formed on the silicon wafer prior to forming the accelerometer, and the accelerometer is not fabricated from the silicon wafer, but rather must be fabricated onto the wafer by depositing polysilicon onto the wafer.
U.S. Pat. No. 5,357,803 also discloses a multi-layered accelerometer where a polysilicon structure is formed above a sacrificial layer of oxide formed on the surface of a silicon wafer substrate. This accelerometer suffers from the same problems discussed in the previous paragraph.
Capacitive transduction has limitations of its own, the most important being parasitic capacitance in the sense element and signal conditioning electronics. Since micromechanical devices are of very small size, the capacitance of the transducer is inherently quite small, and therefore any parasitic capacitance elsewhere can be a significant factor in the overall device performance. Many times, the parasitic capacitance is of the same magnitude as the signal itself, reducing signal strength by 50% or more.
Capacitive transducers are also sensitive to electromagnetic interference, a factor which must be accounted for in the signal conditioning electronics. Signal conditioning electronics for capacitive transducers is in general more complex than that required for piezoresistive transducer schemes. Because of these shortcomings and problems, capacitive accelerometers have not been employed in demanding applications requiring ruggedness, reliability, accuracy, long device lifetime and low cost, e.g., automobiles.
Conventional bulk and surface micromachined accelerometers clearly demonstrate the need for microelectromechanical replacements for conventional large accelerometers in automotive applications. While surface micromachining along with capacitive transduction has alleviated many of the problems associated with bulk micromachining and piezoresistive transduction means, it has, however, introduced a new set of problems which limit device performance and increase manufacturing costs. Thus, conventional microelectromechanical accelerometers are not suitable for use in a vehicle, or like applications in which the accelerometer must be characterized by small size, little weight, low cost, linear and in-phase performance, high stiffness, high capacitance, axis of operation in the plane of the plane of the substrate, self testability, or the like. The foregoing shortcomings of conventional accelerometers exist, despite the prediction approximately fifteen years ago that MEMS offered the ability to fabricate such a device.
Accordingly, what is needed is a microelectromechanical accelerometer with superior price-performance characteristics to be applied to automotive applications. In particular, it would be desirable to provide a small, low-cost accelerator capable of providing a strong linear output signal with little phase-shift and which is fabricated in or from a single silicon substrate.
What is also needed is a new process for fabricating microelectromechanical systems (MEMS) which can produce accelerometers capable of meeting the performance requirements of the automotive applications while being manufactured at very low cost. Such a MEMS fabrication technology should be capable of producing an integrated micromechanical and microelectronic device on a single chip with high yield in manufacturing.
What is also needed is a microelectromechanical accelerometer with high aspect ratio structures that are capable of withstanding high impact forces and which are resistant to out-of-plane and transverse motion. It would be desirable if such high aspect ratio structures could be utilized to create accelerometer devices with high capacitance for capacitive transduction of acceleration. It would be desirable if such devices could be fabricated from single crystal silicon material for improved mechanical and electrical properties. It is also an object that these devices have a capacity for self testing in a preferred embodiment.
Further, what is needed is a technology for microelectromechanical accelerometers which can satisfy not only the present needs for airbag deployment systems, but also projected future automotive needs including side impact, anti-lock braking, electronically controlled suspension, collision avoidance, inertial navigation, or the like.